Coalescing filter element

ABSTRACT

A coalescing filter element for removing liquid droplets from a gas stream comprises a wall which is made of a coalescing filtration material and which defines a hollow space within it. An end cap at one end of the element has a port in it through which gas is supplied to the hollow space to flow through the wall of the filtration material. The end cap has a peripheral portion which engages the element wall and a tube which extends into the hollow space defined by the element wall, so that the port in the end cap comprises an inner opening defined by the tube and at least one peripheral opening located between the tube and the peripheral portion of the end cap. The tube extends beyond the peripheral opening(s) so as to deliver gas to a region of the element wall which is remote from the end cap.

This invention relates to a coalescing filter element for removingliquid droplets from a gas stream.

Filtration of gas in a compressed gas stream is generally required sothat the gas is sufficiently clean for a subsequent application, or tominimise adverse effects of impurities on components of the system. Forexample, removal of compressor oil can be required to minimise chemicalcontamination and accumulation on valves which might lead to malfunctionof the valves, and removal of particulate solid material can be requiredto minimise abrasion.

Coalescing filters are used to collect liquid that is entrained in a gasstream by causing aerosol droplets of the liquid to coalesce and collectas drops, which can flow as a liquid. They generally comprise severallayers of filter media. The density and thickness of the media layersare selected according to the flow rate of the gas stream, the level andnature of the impurities in the gas stream, the level of impurity thatis sought in the gas stream after filtration and so on.

Common filter constructions comprise a tubular filter element mounted ina tubular housing. The gas to be filtered passes radially through thewall of the filter element. Solid particles entering the filter housingare collected by the filter element. Liquid droplets, possibly asaerosols, entrained in the gas are collected by the filter element. Thedroplets can coalesce to form drops, which then collect at the base ofthe filter element for drainage. Clean gas can then be discharged fromthe filter.

Coalescing filter elements of this type can be arranged so that gas tobe filtered flows radially inwardly through the filter media whichprovide the wall of the element. The gas is supplied to the cavityaround the element, between the element and the wall of the housing. Itthen passes inwardly through the element for discharge from the spacewithin the element to the end use application.

It is more common for coalescing filter elements to be arranged so thatgas to be filtered flows radially outwardly through the element wall:the gas is then supplied to the cavity within the element and passesoutwardly through the element wall for discharge from the space betweenthe outer surface of the element and the wall of the housing. Elementsof this latter kind are sometimes referred to as “in-to-out” filterelements, reflecting the direction of flow of gas through the filtermedium.

The present invention is concerned with an in-to-out filter element inwhich gas to be filtered is supplied to the cavity within the filterelement through a port in an end cap, in which the port comprises aninner opening defined by a tube and at least one peripheral opening withthe tube extending beyond the peripheral opening(s) so as to deliver gasto a region of the element wall which is remote from the end cap.

Accordingly, in one aspect, the invention provides a coalescing filterelement for removing liquid droplets from a gas stream, which comprisesa wall which is made of a coalescing filtration material and whichdefines a hollow space within it, and an end cap at one end of theelement which has a port in it through which gas is supplied to thehollow space to flow through the wall of the filtration material, theend cap comprising a peripheral portion which engages the element walland a tube which extends into the hollow space defined by the elementwall, so that the port in the end cap comprises an inner opening definedby the tube and at least one peripheral opening located between the tubeand the peripheral portion of the end cap, with the tube extendingbeyond the peripheral opening(s) so as to deliver gas to a region of theelement wall which is remote from the end cap.

The filter element of the invention has the advantage that it enablescontaminant material which is entrained to flow in the gas stream to bedistributed more evenly through the length of the filter element.

The tube which defines the inner opening can be supported by means ofone or more formations which extend between it and the peripheralportion of the end cap. Preferably, the or each formation has anappreciable axial extent so that the tube is supported by means of atleast one vane which extends between it and the peripheral portion ofthe end cap. It can be preferred for the tube to be arrangedsubstantially centrally in the end cap.

Preferably, the end cap includes at least three vanes, more preferablyat least four support formations, for example at least six vanes or atleast eight support formations extending between the tube and theperipheral portion of the end cap. It will generally be preferred forthe support formations to be arranged symmetrically around the axis ofthe element. (The axis of the element will usually be defined by thegeneral direction of flow of gas through the port in the end cap, andthe filter element will often be rotationally symmetrical about thataxis.)

Peripheral openings in the end cap, between the tube and the peripheralportion of the end cap, can be separated around the end cap by thesupport formations. For example, when there are three supportformations, there can be three peripheral openings. Each of theperipheral openings can then have a radial extent of approximately 120°(when the support formations are thin, which will be preferred in orderto minimise resistance to flow of gas).

Preferably, the ratio of the length of the tube measured from the edgeof the element wall where the end cap engages the wall, to the overalllength of the wall, is at least about 0.1, preferably at least about0.25, for example at least about 0.4.

Preferably, the ratio of the area of the inner opening in the port(defined by the tube) to the total area of the peripheral opening (oropenings) is at least about 0.25, preferably at least about 0.4.Preferably, the said ratio is not more than about 0.75, more preferablynot more than about 0.6.

Preferably, at least one vane in the end cap is configured to impart ahelical flow (relative to the axis defined by the port) to gas flowingthrough the port in the end cap into the hollow space defined by thewall. Such a vane can be provided in the or at least one peripheralopening in the end cap. Such a vane can be provided in the tube whichextends into the hollow space. It appears that the helical flow that isimparted to gas entering the filter element can lead to a more evendistribution of contaminant material in the gas stream over the lengthof the filter element: particles and droplets of contaminant materialwill generally tend to be relatively heavy, and the helical flow canresult in them being directed further into the filter element than hasbeen the case in known elements. Furthermore, primary separation ofliquid droplets from the gas stream can be facilitated as a result ofthe helical flow of gas entering the element. This can facilitatecollection of aerosol liquid droplets to form drops on the surface ofthe coalescing filter media, so that the droplets then collect withinthe media and flow to the base of the element. These benefits in termsof improved filtration efficiency are significant. Furthermore, it hasbeen found that the operating lifetime of a filter element is notaffected adversely by localised collection of entrained liquid dropletson the wall of the filter element close to the inlet port.

When a helical flow is imparted to gas flowing through a central tube inan end cap, this can be achieved by means of at least one vane in thetube. Preferably, there are at least two vanes, more preferably at leastthree vanes, for example at least four vanes. When there are two or morevanes, they can be arranged symmetrically around the axis of the tube,preferably such that they meet at the axis.

The nature of a helical flow which is imparted to gas entering thehollow space within the tubular element can be characterised as atwisted flow. It can also be characterised as a cyclonic gas flow. Acharacteristic of the flow that is imparted to the gas is that, insteadof flowing in a direction which is parallel to the element axis, themovement of the gas includes a component which involves the gas flowingaround the said axis.

A further advantage of imparting a helical flow to gas entering thehollow space within the tubular element is that the reaction of theelement to the helical flow imparted by the vanes can be relied on tominimise the risk of the filter element becoming detached from itshousing. This advantage applies in particular for example when thefilter element is fitted into its relevant housing part by a twistingaction, for example relying on threaded or bayonet formations.

Preferably, at least some of the vanes, generally each of the vanes, hasa twisted non-planar configuration. However, it is envisaged that ahelical flow can be imparted to gas entering the hollow space within thetubular element using vanes which are planar, but arranged so that theplane of each vane is inclined to the axis of the element. Preferably,the angle between each vane and the axis of the element, when theelement is used in cross-section, is at least about 3° more preferablyat least about 5°, for example at least about 10°. The said angle willgenerally be less than about 60°, preferably less than about 50°,especially less than about 30°, for example less than about 20°. It hasbeen found that useful helical flow can be obtained with vanes in whichthe angle of inclination relative to the element axis is small. When thevanes have a twisted non-planar configuration, the angle of inclinationof the vane to the axis of the element is measured with reference to theinclination of a straight line which represents the best fit alignmentof the vane at approximately its mid point. A filter element in which ahelical flow is imparted to gas flowing through the end cap is thesubject of a patent application filed with the present applicationentitled A Coalescing Filter Element and bearing the agents' referenceP11700. Subject matter disclosed in the specification of thatapplication is incorporated in this specification by this reference.

The coalescing filtration material that is used in the filter element ofthe present invention will be selected according to the nature of thegas that is being filtered, the nature of the contaminants (liquiddroplets, aerosols, solid particles etc) to be filtered from the gas,the pressure differential across the filter and so on. Examples of mediamaterials which can be used in the filter element include foamedpolymers (such as polyurethane and polyesters), glass and borosilicatefibre materials, polymeric materials such as polyolefins (especiallypolyethylene and polypropylene) especially in the form of fibres, paperbase materials and so on. Examples of suitable features for the filterelement of the invention can be found in filter elements sold by DomnickHunter Limited under the trade mark OIL-X.

The filter element of the invention can be used to separatecontaminants, especially liquid carried as aerosol droplets, in apressurised gas medium. The filter element can be used in particular toseparate compressor oil droplets from compressed gas, for example inrefrigeration equipment.

Liquid which is collected from the gas stream following coalescence ofaerosol droplets will collect within a housing for the filter element ata low point therein. The housing will generally include a device at itslow point which allows liquid which has collected to be removed from thehousing. The drain should preferably be configured to facilitatedrainage of collected liquid without loss of pressure within thehousing. An example of a suitable drain device is disclosed inEP-A-81826. Generally, the filter element of the invention will bearranged to operate with its axis substantially vertical. The end capwith the port in it for gas to enter the hollow space will be a top endcap. Generally, the filter element will be closed at its base by meansof a base end cap to preserve the pressure differential across theelement, and to prevent by-pass flow of the gas stream between thehousing inlet and outlet.

It can be preferred for the base of the filter element to include araised central portion so that liquid which collects on the inlet sideof the element (before gas introduced to the space within the elementpasses through the element wall) drains to the base thereof, to collectaround the edge of the base, at the bottom edge of the element wall.This can have the advantage of allowing the liquid to settle, reducingthe risk of re-entrainment of liquid in gas as it flows through theelement wall. Liquid which collects within the element and the bottomedge of the element wall can drain through the element wall to flow withliquid which coalesces within the element to the base of a housing forthe element. The provision of a raised central portion in the base ofthe element can also encourage helical movement of gas within theelement, around the axis of the element.

The end cap with the inlet port for gas which is to be filtered can bemade from polymeric material, especially by moulding. A base end capwhen present can be made from polymeric material, especially bymoulding. Metallic materials can also be used for the or each end cap. Asuitable polymeric material will be selected for its chemicalresistance. It should also be able to withstand stresses to which it isexposed during manufacture and use. Suitable polymeric materials willgenerally lend themselves to manufacturing techniques involvingmoulding. Examples of suitable polymeric materials include polyolefins,polyesters, polycarbonates.

The filter element of the present invention can be made by conventionaltechniques which are used to make products of this kind. It will beimportant for appropriate fluid-tight seals to be formed between the endcap and the element wall provided by the filtration material. Forexample, suitable seals can be provided by one or more of mechanicalconnections (for example interference fit) and the use of bondingmaterials such as adhesives. Such techniques are used commonly in theconstruction of coalescing filter elements, for example as sold byDomnick Hunter Limited under the trade mark OIL-X.

The dimensions of the filter element will be selected according to theintended application. Elements according to the invention can be madewith a range of sizes and configurations, including a range of aspectratios (ratio of height to transverse dimension).

Embodiments of the present invention will now be described by way ofexample with reference to the accompanying drawings in which:

FIG. 1 is a cut away view from one side of a filter element according tothe present invention.

FIG. 2 is a cut away view from one side of a top end cap of anotherfilter element.

Referring to the drawings, FIG. 1 shows a filter element 2 whichcomprises a cylindrical wall 4 constructed a suitable filter medium. Thefilter medium will be selected according to the requirements on thefilter when in use, for example in terms of the nature and quantity ofthe impurity (for example as to whether it comprises liquid impurity orsolid impurity or both) in the gas stream, the degree of filtrationrequired of the medium, the pressure to which the assembly is exposedwhen in use. When the impurity to be collected includes liquid (whichwill generally be present as an aerosol for example of compressor oil),the filter medium will preferably be capable of causing liquid dropletsto coalesce. Materials suitable for use in a coalescing filter elementare known, including those sold by Domnick Hunter Limited under thetrade mark OIL-X. Suitable materials include borosilicate and otherglass fibres, activated carbon materials, activated silica materials andso on.

The filter element is closed at its bottom end by means of a bottom endcap 12. The bottom end cap has a raised central portion 14, defining anannular recess for collection of liquid around the base end cap at thebase of the element wall 4.

The element includes a top end cap 16 which has a port 18 in it throughwhich gas can enter the hollow space 20 defined by the element wall. Theport comprises a central tube 22 which is supported in the port by meansof four equally spaced vanes 24 (of which three are visible) within it,each extending between the central tube 22 and the peripheral portion 26of the end cap. Each of the vanes is arranged so that it is aligned withthe axis 30 of the device. The flow of a compressed gas in to the hollowspace 20 is split between the central tube 22 and peripheral openings 28which are defined by the central tube 22, the peripheral portion 26, andthe vanes 24.

The top end cap is connected to the element wall provided by thefiltration material 4 mechanically by fitting the filtration materialinto the groove 32 in the top end cap. The filtration material can beheld in that groove by means of, for example, an interference fit andthe use of an adhesive material. Similar connection techniques can beused to fasten the bottom end cap to the element wall.

The top end cap can include formations to enable a connection to be madebetween it and a housing for the element. For example, the top end capcan be formed with a male thread or with appropriate bayonet rampformations, commonly to provide the male part of a threaded or bayonetconnection. For example, the filter element can be fitted into anopening in a pressurised gas distribution plate, or in a cylindricalhousing.

In use, the filter element is installed in an appropriate housing towhich it is sealed so that a pressure differential is maintained acrossthe wall of the element. Gas is supplied to the hollow space 20 withinthe filter element through the port 18 in the top end cap 16. Gasflowing through the central tube 22 of the end cap 16 tends to beprojected further into the hollow space 20 than gas flowing into thehollow space through the peripheral openings 28. This can aiddistribution of contaminants across the available surface area of thefiltration material.

Gas flows through the filter element wall 4 provided by the filtrationmaterial. Particles within the gas stream are retained within thefiltration material. Liquid droplets collect within the filtrationmaterial and coalesce. They then flow down the element wall and collectin the base of the element. Clean gas is then collected from the spacearound the tubular element for supply for an end use application.

FIG. 2 shows an alternative construction of top end cap for a filterelement. The end cap comprises a central tube 40 and a peripheralportion 42, and the central tube is held relative to the peripheralportion by a plurality of transversely extending vanes 44. The centraltube, the peripheral portion of the end cap and the transverselyextending vanes define peripheral openings 45. The central tube alsocontains vanes 46 which are spaced uniformly around the axis 48 of theend cap and which are joined to one another on the axis. Each of thevanes which define the peripheral openings 45 and in the central tube 40is inclined to the axis 48 of the end cap so that gas flowing over thevanes has a cyclonic twisting movement imparted to it.

1. A coalescing filter element for removing liquid droplets from a gasstream, which comprises a wall which is made of a coalescing filtrationmaterial and which defines a hollow space within it, and an end cap atone end of the element which has a port in it through which gas issupplied to the hollow space to flow through the wall of the filtrationmaterial, the end cap comprising a peripheral portion which engages theelement wall and a tube which extends into the hollow space defined bythe element wall, so that the port in the end cap comprises an inneropening defined by the tube and at least one peripheral opening locatedbetween the tube and the peripheral portion of the end cap, with thetube extending beyond the peripheral opening(s) so as to deliver gas toa region of the element wall which is remote from the end cap.
 2. Afilter element as claimed in claim 1, in which the tube which definesthe inner opening is supported by means of at least one vane whichextends between it and the peripheral portion of the end cap.
 3. Afilter element as claimed in claim 2, which comprises at least threevanes extending between the tube and the peripheral portion of the endcap.
 4. A filter element as claimed in claim 2, in which the vanes arearranged so that they imparts a helical flow to gas flowing through theperipheral openings, relative to the axis defined by the port.
 5. Afilter element as claimed in claim 1, in which the tube is locatedapproximately centrally in the inlet port.
 6. A filter element asclaimed in claim 1, in which the ratio of the length of the tubemeasured from the edge of the element wall where the end cap engages thewall, to the overall length of the wall, is at least about 0.1.
 7. Afilter element as claimed in claim 1, in which the ratio of the area ofthe inner opening in the port to the total area of the peripheralopening (or openings) is not more than about 0.6.
 8. A filter element asclaimed in claim 1, in which the ratio of the area of the inner openingin the port to the total area of the peripheral opening (or openings) isat least about 0.25.
 9. A filter element as claimed in claim 1, in whichthe tube contains at least one vane within it for imparting a helicalflow to gas flowing through the tube, relative to the axis of the tube.